The present invention relates generally to optoelectronics involving optical components of significantly different cross-sectional areas, such as optical fibers joined to optical elements such as lenses, filters, gratings, prisms, and the like, and, more particularly, to fiber collimators.
Fiber collimators find extensive use in optoelectronics, particularly in the coupling of light from (to) an optical fiber to (from) a collimating lens. Fiber collimators are basic components of telecommunication products such as isolators, mechanical switches, couplers, circulators, optical switches, and wavelength division multiplexers. Such fiber collimators are fabricated by joining an optical fiber to an optical element.
While splicing of one optical fiber to another or of one optical fiber to an optical waveguide is known, the sizes are similar and localized heating can be used to fusion-splice the optical components together. Splicing an optical fiber to a much larger optical element is more challenging. For example, U.S. Pat. No. 4,737,006 entitled xe2x80x9cOptical Fiber Termination Including Pure Silica Lens And Method Of Making Samexe2x80x9d, issued to K. J. Warbrick on Apr. 12, 1988, discloses fusion-splicing an undoped (pure) silica rod to a single mode fiber to fabricate a collimator, employing an electric arc. However, this is an extremely complicated method and has limited applications.
The most often utilized processes for attaching optical fibers to the larger optical elements include (1) bonding the fiber faces directly to the optical element with adhesives or (2) engineering a complex mechanical housing which provides stable positioning of air-spaced fibers and optical elements throughout large changes in environmental conditions.
The use of adhesives in the optical path of such devices is undesirable due to the chance of degradation of the adhesive over time. On the other hand, spacing the fibers a fixed distance away from the optical elements by utilizing complex mechanical housings requires the use of anti-reflection coatings at all air-glass interfaces in order to minimize losses of optical energy through the device. The presence of air-glass interfaces also provides a source of back-reflected light into the optical fibers. This phenomenon, known as back-reflection, is a source of noise in many communications networks, and effectively limits transmission bandwidth of such communications networks.
The parent application to the present application provides a simple process for fusion-splicing two optical components of different sizes together, e.g., fusion-splicing an optical fiber to a much larger (at least 2xc3x97 diameter) optical element, using laser heating.
While butt-end coupling of the optical fiber to an optical element is desired for simplicity, in fact, the prior art requires angle-cleaving of the optical fiber and angle-polishing of the optical element to reduce or minimize back-reflection. That is to say, the optical fiber and optical element are both processed to provide coupling at a non-perpendicular angle to the optic axis, which is parallel to the optical fiber. Back-reflection resulting from simple butt-end coupling reduces the optical output and efficiency. In optical communication systems, back-reflection also has a detrimental impact on the BER (bit error rate) and the SNR (signal-to-noise ratio). Due to its uncontrolled generation and propagation, power reflected back in the fiber is considered excess noise when detected.
In previous art, it has been shown that positioning an angle-cleaved fiber or angle-polished fiber in proximity to the angle-polished face of a collimating lens results in excellent collimation and excellent performance characteristics of fiber collimators. However, these existing technologies for assembling collimators require very labor intensive active alignment techniques. The alignment techniques include manipulating the position of the fiber relative to the lens in three linear axes and three rotational axes during final assembly. If a collimator can be built that effectively makes the fiber and the lens a single piece, then alignment can be reduced to two linear and two rotational axes during the fusion process and there is no need for alignment during final assembly, thereby reducing costs dramatically.
A key performance parameter to be minimized in collimator assemblies is back reflection of light down the fiber. By butt-coupling or fusion-splicing a fiber to a lens of the same refractive index, there is no apparent interface to cause back reflection. The beam is then allowed to diverge in the lens and does not see an index break surface until it exits the lens. By then, the beam is so large that the amount of light that can return to the fiber core is extremely small.
The laser fusion-splicing method disclosed and claimed in the parent application to the present application provides a back-reflection of xe2x88x9257 dB. This may be acceptable for some applications. However, a further reduction in back-reflection would be desirable for other applications.
In accordance with the present invention, a fiber collimator is provided, having reduced back-reflection, improved pointing accuracy, and improved power handling characteristics. The fiber collimator comprises at least one optical fiber fusion-spliced to an optical element, such as a collimator lens. The optical element is constructed from an optical material which has an index of refraction nearly equal to the index of refraction of the optical fiber to which it is fusion-spliced. For commercial reasons, pure fused silica glass is preferred. In addition, the fiber collimator may comprise at least one fiber fusion-spliced to an optical element other than a collimator lens, such as a plano-plano xe2x80x9cpelletxe2x80x9d which is subsequently assembled jointly with a separate collimator lens. This latter configuration is especially useful in creating collimators with long optical path lengths and associated large collimated beam diameters. The utilization of an optical pellet provides all the advantages of reduced back-reflection and improved power handling while reducing the required lens thickness in long focal length collimators.
The splice created by the laser fusion-splice process will typically have back-reflection of xe2x88x9257 dB. This slight residual back-reflection is due to the small refractive index difference between the fiber core and pure fused silica. Even lower back-reflection can be achieved by creating a thin axial gradient layer at the splice junction. Simple adjustment of the fusion-splice process parameters is enough to favor the creation of such an axial gradient through the diffusion of the dopant in the fiber core. The resulting back-reflection can be less than xe2x88x9265 dB with no detrimental effect on the quality of the splice. Similar results can be obtained with a prior doping of a thin surface layer on the optical element to be fused.
By attaching fibers directly to other optical components without using epoxies or special termination techniques, costs are reduced, environmental stability is improved, alignment accuracy is enhanced, pointing accuracy is improved, and power handling is significantly increased.
Other objects, features, and advantages of the present invention will become apparent upon consideration of the following detailed description and accompanying drawings, in which like reference designations represent like features throughout the FIGS.